Rare-earth elements have been widely used in applications such as a phosphor, a magnetic substance, an abrasive, and a catalyst. Particularly in the magnetic substance, the use of the rare-earth elements as materials for a permanent magnet has been rapidly expanding because a magnet having a large maximum energy product and a large residual magnetic flux density can be obtained by adding the rare-earth elements to transition elements. For example, Patent Literature 1 discloses materials for an Nd—Fe—B-based permanent magnet having an excellent maximum energy product and an excellent residual magnetic flux density. In addition, Patent Literature 2 discloses a technology for improving the thermal stability of magnetic characteristics, which is a defect of the Nd—Fe—B-based permanent magnet, by substituting part of Nd with Dy in the permanent magnet.
For example, ores such as monazite, bastnaesite, xenotime, and ion adsorption clay mineral are used as raw materials for such rare-earth elements. The rare-earth elements are caused to leach from any of these ores by using an acid aqueous solution, for example, an aqueous solution of a mineral acid such as sulfuric acid, and the rare-earth elements are separated and collected from the resultant leachate. However, these ore resources are unevenly distributed on the earth, and the abundance of each element in the rare-earth elements significantly varies for each ore. In particular, there are very few mines in which ores containing heavy rare-earth elements having atomic numbers of 64 to 71 and having high mine profitability can be mined, and it is concerned that the depletion of the resources of Dy, which is in especially high demand, may occur.
On the other hand, the rare-earth elements are also contained in bauxite, which exists as a resource abundantly and which is an ore resource of aluminum. It is known that the rare-earth elements are caused to dissolve from bauxite and are then separated and recovered (see, for example, paragraph 0003 in Patent Literature 3). Further, it is also known that rare-earth elements are caused to leach by using, as a raw material, a solid residue produced as a by-product in a Bayer process and remaining after the collection of aluminum in the production of aluminum from the bauxite through the steps of the Bayer process and a Hall-Héroult process (The solid residue is hereinafter referred to as “bauxite residue”. A bauxite residue containing Fe2O3 as a main component has a red color and is generally called “red mud”.) and are then separated and recovered (Patent Literature 4).
In addition, the rare-earth elements are stable in an alkaline aqueous solution by taking the forms of compounds such as oxides and hydroxides, and the compounds do not react with a solution of sodium hydroxide even when the solution is heated and pressurized. Thus, in the bauxite residue, the rare-earth elements are to be concentrated to the extent corresponding to the amount of the aluminum component caused to leach with the solution of sodium hydroxide in the Bayer process described above. According to studies of the inventors of the present invention, the bauxite residue contains rare-earth elements about three times on the average in comparison to the content of rare-earth elements in the bauxite. Further, the bauxite residue is an industrial waste which is produced in the production of aluminum from bauxite, and is stably produced as a by-product in the production of aluminum, and hence can be easily obtained. Therefore, the bauxite residue is expected to be used as a raw material for rare-earth elements.
However, detailed examination of Patent Literature 4 above has revealed that, as described in Examples 1 and 2 thereof, a bauxite residue containing, in a dried state, 52.0% of Fe2O3, 6.5% of TiO2, 18.0% of ignition loss, 12.9% of Al2O3, 2.4% of SiO2, 1.6% of Na2O, 5.0% of CaO, and 0.6% of P2O5 is used as a raw material, and a leaching operation (leaching, or digestion or maceration) is repeated two or three times at 10 to 70° C. by using a sulfurous acid solution having a high pH value and a sulfurous acid solution having a low pH value to adjust the final ph value to 1.35 to 2.4. Accordingly, rare-earth elements are caused to leach while keeping the dissolution of Fe and Ti contained in the bauxite residue at a low level, and the rare-earth elements are then separated and recovered by using a solvent extraction method. In this case, although 50 to 85% of the content of Y in the bauxite residue are caused to leach and the leaching ratio of Dy is not described, in a leaching time of 20 minutes, which is considered to be preferred to almost saturate the leaching amount of the rare-earth elements without continuously increasing the leaching amount of Fe, the leaching ratio of Nd is lower than that of Y and is only about 58% (see the descriptions on lines 32 to 36 in column 7, Tables 1 to 3, and FIG. 2 in Patent Literature 4).
That is, the technology described in Examples 1 and 2 of Patent Literature 4 involves repeating the leaching operation two or three times, and hence, as the amount of a leachate increases, the cost of the leaching step increases at the time of causing rare-earth elements to leach from a bauxite residue because, for example, it is required to repeat solid-liquid separation two or three times. Moreover, the liquid-solid ratios at the time of the leaching operations are set to 4:1 and 10:1 in digestion or maceration carried out twice in Example 1 (see Table 1) and set to 4:1 and 8:1 in digestion or maceration carried out twice in Example 2 (see Table 3). Accordingly, the amount of a leachate becomes 14 times or 12 times the amount of the bauxite residue serving as a raw material. Thus, this technology has a problem in that a solvent extraction apparatus in the separation step of separating and recovering rare-earth elements from the leachate by the solvent extraction method is increased in size and the cost thereof is also increased.
By the way, the inventors of the present invention used 0.102 kg of a bauxite residue having the same composition as that of the bauxite residue used in Examples to be described below, and followed the experiment in Example 1 of Patent Literature 4, which involves using an aqueous solution of sulfurous acid as an acid aqueous solution and repeating the same leaching operation three times under the conditions of a liquid-solid ratio (L/S) of 5.0, a temperature of 30° C., a pressure of 0.1 MPa, and a time of 15 minutes. The results are as shown in Table 1. In the first leaching operation, the leaching ratio of Y merely reached 5 mass % or less, and the total leaching ratio of Y additionally including the leaching ratios of the second and third leaching operations was 52 mass %. However, the leaching ratios of Nd and Dy merely reached 41 mass % and 43 mass %, respectively, which were merely even lower values in comparison to the leaching ratio of Y.
TABLE 1Usage of bauxite residuekg0.102FirstKind of acidH2SO3leachingLiquid-solid ratio5.0LeachingTemperature° C.30conditionspHAfter completion 3.27of leachingTimeMinutes15SecondKind of acidH2SO3leaching Liquid-solid ratio5.0LeachingTemperature° C.30conditions pHInitial stage 2.05of leachingAfter completion 3.20of leachingTimeMinutes15ThirdKind of acidH2SO3leaching Liquid-solid ratio5.0LeachingTemperature° C.30conditions pHInitial stage 1.21of leachingAfter completion1.82of leachingTimeMinutes15pH valueInitial stage of leaching3.3After leaching1.2Leaching Y52ratioNd41(mass %)Dy43Ca88Al40Si99Ti0.3Fe0.2